The use of electrographic processes to produce images is well known. In these processes, an electrostatic latent image is produced directly by "spraying" charge onto an accepting dielectric surface in an imagewise manner. Styli are often used to create these image patterns and are arranged in linear arrays across the width of the moving dielectric surface. The latent image is then developed on the dielectric substrate with suitable toner(s).
The term "electrography" is clearly distinguishable from "electrophotography" to those skilled in the art. As explained above, "electrography" is a process for producing images by addressing an imaging surface, normally a dielectric material, with static electric charges (e.g., as from a stylus) to form a latent image which is then developed with suitable toner(s). The term is distinguishable from "electrophotography" in which an electrostatic charge latent image is created by addressing a photoconductive surface with light. The term "electrostatic printing" and the like is commonly used in the literature and appears to encompass both electrography and electrophotography.
The electrographic process is often used to produce large size prints, which are commonly exhibited outdoors in some fashion, such as a sign on the side of a truck or a building. At the end of the electrographic process where the image has been developed on the dielectric substrate, the produced print is often enclosed between two layers of clear, vinyl plastic film and is used directly as an outdoor sign. It has been found, however, that the properties of the dielectric imaging sheet are frequently not suitable for the final image. The typical dielectric paper substrates lack the water resistance required for outdoor signs and more resistant substrates such as polyvinylchloride and polyvinylacetate films cannot be imaged directly because of their electrical and mechanical properties.
Electrophotographic processes are not practical for making large prints because the size of the typical equipment used in the electrophotographic process is rather small and therefore, the equipment cannot accommodate the production of large size prints.
In the electrophotographic process, silicones are sometimes employed as release layers on photoconductors. The silicone release layer serves to enhance the transfer of a toner image from the photoconductor to a support surface, such as a polymeric film. SYL-OFF.TM. 7610 silicone release agent (Dow Corning Corporation) is commonly used on photoconductor plates.
Because it would be desirable to have large outdoor signs which would be more weather resistant than the conventional ones discussed herein earlier, the desirability of transferring a produced image from a dielectric substrate to a more weather resistant receptor substrate, such as one composed of a thermoplastic polymer, has been recognized. However, until now, there have been no suitable release coatings available for dielectric substrates which work very well in the electrographic process.
Even though silicones have been used as release coatings for photoconductive processes, they do not perform well as release coatings in electrographic processes. To begin with, the cross-linked or dried silicone release coating can leach out into the toner reservoir thereby contaminating the toner. Furthermore, it has been found that the dried toner does not adhere very well to silicone release surfaces and therefore can be rubbed off during the electrographic printing process.
In the process of trying to find a release coating which would work well in the electrographic processes by overcoming the above disclosed disadvantages of silicone release coatings, Applicants discovered that silicone-urea block polymers make a very effective release coating for dielectric substrates. They do not leach out into the toner solution and contaminate it. Furthermore, the silicone-urea block polymers offer adequate release characteristics that afford good transfer of a developed image from a dielectric substrate to a durable thermoplastic receptor substrate.
The use of silicone-urea block polymers as release coatings for pressure sensitive adhesives is known; see European Patent Application No. 0/250,248. Pressure sensitive adhesives, however, are fundamentally different from dielectric coated substrates since pressure sensitive adhesives are not employed in electrostatic printing processes. In fact, the function of a release coating in a pressure sensitive adhesive is entirely different than the function of a release coating in an electrostatic printing process even when the same type of polymers are employed in the coatings. In a pressure sensitive adhesive, the release coating acts primarily as an anti-stick agent. In an electrographic process, the release coating must function to both retain toner during the charge deposition and image developing processes as well as to transfer the produced image effectively to a receptor substrate. Also, the release coating for a dielectric substrate, as opposed to one for a pressure sensitive adhesive, must be very thin, e.g., 0.05-2.0 microns, so that the dielectric properties of the substrate are not affected and high density images are achieved. Thus, the properties of release coatings in an electrographic process must be very different compared to the properties of release coatings in a pressure sensitive adhesive. This is readily apparent when one considers that whereas pure silicone polymers find acceptable use as release coatings in pressure sensitive adhesives, they do not find acceptable use as release coatings for dielectric substrates for the reasons given earlier herein.
Applicants' discovery that silicone-urea block polymers make very effective release coatings on dielectric substrates will be very important to the printing industry. For the first time that Applicants are aware of, large quality prints will now be available for use as outdoor signs without having to worry about the perennial problem of weather resistance of the substrate the image is produced on.